DHCP messages are really BOOTP so lets name the structure accordingly.

DHCP messages are really BOOTP so lets name the structure accordingly.
While here, stop making assumptions about the size of a DHCP packet based
on a fixed structure from a default MTU of 1500.
Instead, make it flexable and set the DHCP packet size to the interface MTU
less UDP and IP headers.
As a result, we need to know the size of the DHCP packet received
rather than just walking the old fixed DHCP message structure.

This makes Coverity happy about tainted scalar values when parsing DHCP
messages.